http: do not allow multiple instances of certain response headers

Response headers such as ETag and Last-Modified do not permit
multiple instances, and therefore the comma-separated syntax is
not allowed. When multiple values for these headers are specified,
use only the first instance.

Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/node/pull/3090